In manufacturing processes which include a step for cooling materials, gas cooling or quenching of materials often is a rate limiting step. A quenching or cooling step in a process can be carried out on a batchwise or a continuous basis. An improvement in the cooling rate in the quenching step generally results in overall productivity improvement for the process.
Such improvements generally can be achieved in two ways: 1) increasing the heat transfer coefficient of the coolant, e.g. by increasing the fluid velocity of the coolant or changing the coolant's thermal properties, or 2) by increasing the temperature difference by lowering the temperature of the cooling fluid. Other limitations for gas quenching include the requirement that the quenchant or coolant gas and the cooling process must limit the heat removal rate to prevent damaging temperature gradients between the surface and the interior of the material being cooled. If temperature gradients develop, they may cause residual thermal strains or non-uniform material properties. In addition, the quenchant gas should not be chemically reactive with the material to avoid creation of undesirable compounds at the material surface which can alter the purity of the original composition of the material.
Both approaches for improving cooling rates, i.e. 1) increasing the heat transfer coefficient and 2) increasing the temperature difference, have been undertaken both independently and jointly. However, most attempts at improving the cooling rate have pursued one method or the other.
Improvement of the cooling rate by increasing the temperature difference may be accomplished by reducing the temperature of a quenchant gas. By employing a cryogenic heat exchanger, the temperature of the quenchant gas is lowered significantly below ambient temperature. Generally this is done by flowing the quenchant gas through a cooling coil immersed in a liquid nitrogen bath prior to the quenching step. This method pre-cools the quenchant gas only to a temperature approaching that of liquid nitrogen. Because of the temperature difference involved for indirect cooling, the quenchant gas would not be quite as cold as could be achieved if the gas were directly cooled in nitrogen.
One example of where both methods have been employed, (i.e. increasing the heat transfer coefficient and increasing the temperature difference), is in processes with vacuum furnace cooling systems. Both a recirculating blower and an external heat exchanger are used to reject the heat captured from a quenchant gas. Either water or air is the cooling fluid used in the heat exchangers and generally reduce the temperature of a quenchant gas only to ambient temperature. Cooling rates are sometimes improved by expensive modifications such as increasing the quench pressure or gas velocity.
The production of optical fibers requires cooling. The fibers are cooled to levels approaching ambient temperature before surface coatings can be applied. Various forms of helium gas systems are used for cooling which enable increased fiber draw speeds and production rates. However, most of the gas cooling systems are once-through flows without quenchant gas recirculation making the process more expensive.
Further, it is well known that helium has a high thermal conductivity and can be used to achieve faster rates of cooling. However, helium is more expensive than nitrogen or argon and thus, the use of pure helium as a quenchant gas is generally not cost effective. It is also known that cooling with mixtures of helium gas can result in improved heat transfer coefficients and therefore faster cooling. In some situations, helium mixtures may provide a higher cooling rate than 100% helium. One article discussing the characteristics of helium mixtures is Gas Quenching With HELIUM, Advanced Materials & Processes, February 1993.
U.S. Pat. No. 5,173,124 discloses the use of mixtures of helium and nitrogen or argon in specific ratios in turbulent flow conditions for improved cooling. Though this reference teaches achieving high cooling rates with the specified mixtures, it does not disclose how these mixtures are accomplished.
A variety of methods may be used to achieve the required mixtures of helium and a cryogen such as nitrogen or argon. U.S. Pat. No. 5,157,957 teaches a complex method for producing controlled gas mixtures which can be used in ultratrace level analysis. Mixing is carried out through a series of piping, valves, regulators and a mixing chamber. This system is sophisticated and costly as it produces very specific mixture compositions that are required to be highly accurate in quantities to be used as trace gas. Such a system would not be cost effective to produce mixtures in large bulk quantities.